In response to its toxicity, cells keep copper concentration under strict control allowing enough metal to be available for protein assembly but below Stem Cells inhibitor damage induction threshold [4]. Current knowledge of copper homeostasis systems in bacteria has been elucidated from the study of gamma proteobacteria such as Salmonella enterica sv. Typhimurium [5], Shigella flexneri[6] and Escherichia coli[7]. In these organisms, the archetypical copper resistance response involves the coordinated function of four different systems: CopA/Cue, Cus, Pco and Cut, responsible for copper import, export or detoxification. A set

of copper-sensing transcriptional regulators (CueR, CusR, CusS, PcoR and PcoS) VX-689 specifically modulate the expression of these genes [8]. For instance, in E. coli under aerobic conditions, CueR activates the expression of copA and cueO, encoding for a periplasmic multi-copper oxidase (MCO). CueR also induces expression of cueP, encoding for a periplasmic protein of unknown function putatively involved in copper-resistance in Salmonella[5]. While CopA pumps out Wnt inhibitor excess copper from the cytoplasm

to the periplasm, CueO oxidizes Cu(I) to Cu(II) in periplasm thereby reducing Cu(I) concentration [9, 10]. Under anaerobic conditions, CusR and CusS activate the transcription of the cusCBAF operon that encodes for a complex that pumps Cu(I) to the extracellular space [11]. This complex consists of the inner membrane pump CusA, the periplasmic protein CusB Casein kinase 1 and the outer membrane protein CusC forming a channel through the periplasm. CusF has been proposed to feed the CusABC channel with copper from the periplasmic space [12]. PcoR and PcoS are transcriptional regulators for the copper-inducible expression of the pcoABCD operon [13]. pcoA encodes for a periplasmic MCO. There is no known

function for PcoB although it may function as an outer membrane protein. PcoC is a periplasmic copper carrier with two metal binding sites selective for Cu(I) or Cu(II) and has been suggested to interact with PcoD (an integral membrane protein) in copper translocation into the cytoplasm. pcoE apparently encodes for a cytoplasmic protein with a putative function as a copper scavenger. There is no information available regarding the regulation of the Cut system that involves at least six proteins: CutA, CutB, CutC, CutD, CutE, and CutF [14]. CutF and CutC have been described as involved in copper tolerance in E.coli. Since CutC is a cytoplasmic protein perhaps involved in intracellular trafficking of Cu(I), while CutF is an outer membrane protein [15], we only included CutF in our analysis Figure 1.